In a video signal tapped off from a video signal source, e.g., a computer, a phase relation between a video signal and a synchronizing signal, i.e., a period between a horizontal synchronizing signal (herein after referred to as H. Sync. signal) and a leading edge of the video signal as well as a period between a vertical synchronizing signal (hereinafter referred to as V. Sync. signal) and a leading edge of the video signal, differs, in many cases, depending on the computer model. A picture location on the LCD thus differs depending on the type of computer.
Several prior art schemes addressed how to adjust the phase relation automatically when the video signal tapped off from the video signal source such as a computer, etc. is displayed on the LCD. One prior art method addressing this problem is disclosed in H07-219486,unexamined Japanese Patent Application Publication.
This prior art compares a phase relation between the video signal from the video signal source sliced at a predetermined level by a comparator with a LCD driving pulse generated from both of the H. Sync. signal and V. Sync. signal from the video signal source by using an AND circuit, and the comparison result is fed back to a central processing unit (CPU). Based on the comparison result, the CPU controls the phase of the LCD driving pulse, whereby a location of the picture on the LCD can be automatically adjusted.
This prior art aims to save time for adjusting and can be used as an adjusting tool such as an adjusting switch for a user to adjust a picture location on the LCD while watching a displayed picture.
The video signal tapped off from the video signal source such as a computer, etc., has various influencing factors other than the period between the H. Sync./V. Sync. signals and the leading edge of the video signal. Examples of the various influencing factors include a period until a trailing edge, scanning timing, horizontal scanning frequency, the number of scanning lines, the number of pixels, the dot clock frequency used in outputting the video signal, all of which may differ depending on the type of computer.
In a video signal tapped off from the video signal source such as a computer and the like, a number of effective pixels within one horizontal period and a number of effective scanning lines within a vertical period, in general, are not identical with a number of effective pixels and a number of effective scanning lines which a LCD can display. When the LCD apparatus simply provides the video signal tapped off from the video signal source with an analog-digital conversion (hereinafter referred to A/D conversion) and transmits the digital RGB signals into the LCD, the picture contained in the signal cannot be properly displayed on the LCD. (Hereinafter, the phrase "just scan" is used for describing a picture which contains a sufficient quantity of a video signal for proper display on an LCD.)
In order to just scan the LCD, the LCD apparatus must make a scan conversion for an input signal so that a number of pixels within one horizontal period and a number of scanning lines within one vertical period of the input video signal are identical with those numbers of the LCD.
To be responsive to such a variety of timing, the LCD apparatus must determine a phase of the LCD driving pulse so that a picture can be displayed at the center of the LCD display area, whereby "just scan", i.e., no conversion, is performed. Further, the LCD apparatus must determine an appropriate scan conversion rate responding to a timing of the signal source. (A scan rate=a number of pixels before conversion vs. that of after conversion, a number of effective scanning lines before conversion vs. that of after conversion.)
The conventional automatic adjustment of a picture location is only effective when the timing of the input signal source, in particular, the horizontal frequency is identical with the horizontal driving pulse which drives the LCD apparatus. In other words, the conventional method is only effective when no scan conversion is necessary. Namely, the conventional method can automatically adjust the picture location, but not adjust a picture size.
Even when the frequency of the driving pulse is the same as the scanning frequency of the input signal source, i.e., when the number of pixels of the input video signal is the same as the number of effective pixels of the LCD, picture quality is sometimes nevertheless lowered depending on a dot clock frequency that the computer uses for generating the video signal.
The dot frequency used for generating the video signal, in general, differs depending on the type of computer.
In order properly to display a picture on the LCD, the dot clock frequency must completely coincide with a LCD sampling clock frequency which is used in A/D conversion.
However, a conventional LCD apparatus does not have an automatic adjuster of the sampling clock frequency used in A/D conversion.
Since a variety of signal-source-timing is available in the conventional type of LCD apparatus, the scan conversion as well as determination of an optimal scan conversion rate is required for "just scan", however, the conventional LCD apparatus, in fact, cannot automatically adjust the picture location and size responding to a signal having an arbitrary timing.
In the conventional LCD apparatus, the dot clock frequency of the signal source cannot coincide with the sampling clock frequency used in the A/D conversion even when the signal does not require scan conversion.
Thus, a user must adjust the picture location, size and the sampling clock frequency while watching the conventional LCD in order to display a sufficient quantity of the video signal generated by various signal sources. This adjustment requires means for adjusting the picture location, size, and sampling clock frequency. As a result, a structure of operation means becomes complicated.